Spinal manipulation is a form of body-based therapy patients often seek for treatment of musculoskeletal complaints. While numerous techniques are used clinically, each shares the common denominator of applying force to the spine. The most form of spinal manipulation includes a short lever, High Velocity, Low Amplitude (HVLA) thrust. Investigation of HVLA spinal manipulation is the focus of this application. By its very nature spinal manipulation is a mechanical intervention that lasts a fraction of a second (typically <200ms) yet produces effects that outlast the intervention itself. How? This question provides the basis for our study and motivates our long term goal: to understand and improve the effective use of spinal manipulation. We will take advantage of an animal model and approaches developed by the two co-leaders in order to understand the relationship between spatial and temporal characteristics of a spinal manipulation and their effects on neural and biomechanical responses from paraspinal tissues. We will determine whether either the responsiveness of primary afferent signaling from paraspinal muscle spindle and/or the passive biomechanical properties of the manipulated region outlast the manipulation itself. Specifically, we will determine if muscle spindle responsiveness increases and spinal stiffness decreases as a function of 1) the spinal manipulation's duration; 2) the presence, magnitude or shape of a preload preceding the manipulation; 3) the anatomical contact point used for the spinal manipulation; and 4) the direction with which the manipulation is applied. The information from these complementary studies will help provide information useful for identifying dosing features of the manipulation to which the nervous system and biomechanical properties of paraspinal tissues may be most responsive and in determining strategies for optimizing the delivery of SM. The study is innovative in that it is represents the first systematic study to investigate this relationship and uses the most commonly applied form of spinal manipulation, a high velocity low amplitude spinal manipulation. The study is significant because it contributes to accomplishing NCCAM's strategic plan by characterizing the biomechanics of manipulative procedures in an effort to clarify the mechanisms of action operative in manipulation practices.